1. Field of the Invention
This invention relates to compositions and processes for depositing zinc phosphate containing conversion coatings on metal surfaces, particularly the surfaces of iron, steel, galvanized steel and other zinciferous surfaces, and aluminum and its alloys that contain at least 45% by weight of aluminum. The invention particularly relates to concentrated compositions containing all the active ingredients required for a working conversion coating composition, including an "internal" accelerator, i.e., an accelerator that is stable when a composition containing the accelerator and all other active ingredients required for a working phosphating composition is stored.
2. Statement of Related Art
The general process of zinc phosphate conversion coating is well known in the art: Contact of active metals with aqueous acidic compositions containing zinc and phosphate ions results in the deposition on the active metal surfaces of a conversion coating containing zinc phosphate. If the active metal is ferrous, iron phosphates are usually included in the coating, and in modern practice nickel and/or manganese are often included in the coating composition and thereby in the coating formed. In order to speed the process and improve the uniformity of the coating, it is customary to include in the coating composition a component called an "accelerator" that does not usually become incorporated into the coating formed. Typical widely used accelerators include nitrate and nitrite ions, chlorate, soluble nitroaromatic organic compounds such as p-nitrobenzene sulfonic acid, and hydroxylamine.
The most widely used accelerator in current commercial practice is believed to be nitrite, but this material suffers from chemical instability, so that it can not satisfactorily be incorporated into concentrated compositions or concentrates that contain most or all of the other ingredients needed for a zinc phosphate conversion coating, are widely available from several commercial sources, and are diluted with water before use to provide a working solution. When nitrite acceleration is desired, a separate addition of nitrite to the working solution must be made, and this is considered inconvenient by many users. A single package concentrate is highly desirable commercially.
It is generally known that some of the ingredients of working zinc phosphate conversion coating forming liquid compositions are consumed by incorporation into the phosphate coatings formed, and in general some ingredients are so consumed in different proportions from those that prevail in the conversion coating forming liquid composition. Therefore, if an initial volume of optimal working composition is used to phosphate extensive amounts of metal surface per unit volume of the initial working composition, at least some of the ingredients will need to be replenished in order for the liquid composition to continue to function as desired. Compositions called "replenisher concentrates" or simply "replenishers" are known for this purpose in the art, and generally contain most if not all of the same ingredients as a concentrate composition to be used as described above in making up a fresh working solution, but often in different proportions. The composition of an optimal replenisher is dependent on a variety of factors, including the metal or mixture of metal types being phosphated, the initial bath composition, the amount of drag-out of phosphating composition into subsequent stages of treatment, and the amount of sludge formed in the phosphating composition.
Accordingly, one object of this invention is to provide a single package concentrate, which may be denoted a "make-up concentrate", that gives, after appropriate dilution, a working composition for zinc phosphate conversion coating that produces coatings at least as high in quality, at a speed of coating that is at least as high, as the coating quality and speed achieved from working compositions with nitrite accelerators, with other ingredients than the accelerator the same or similar to those of the compositions according to this invention. Another object is to provide a single package replenisher concentrate that is suitable for replenishing a working composition as described above, after the latter has been used to coat an extensive area of metal surface compared to the volume of the working composition. Other objects will be apparent from the description below.